The overall objective is to select a known nonpathogenic microbial flora that when implanted in gnotobiotic mice will alleviate the anomalies of germfree life and impart conventional-like characteristics to the previously germfree host animal. SPF animals established by association of germfree mice with a known nonpathogenic flora can provide a useful experimental tool, perticularly if such animals, when put under stress (immunosuppression etc.) were not to succumb to infection from the indigenous flora with pathogenic potential. A number of microbial isolates from the mouse intestinal flora have been associated singly and in combination in germfree mice. Thus far, no single or combination of known organisms have completely alleviated the anomalies of germfree life. Current studies are concentrated on highly oxygen sensitive anaerobes isoated from conventional mice and inoculated orally into gnotobiotic mice. One of these organisms, a fusiform-shaped Clostridium sp., has been successfully established as a monoassociate in gnotobiotic mice.